Electron pair (de)coupling in aniline radical cation and its implications for organic "mixed valence" systems

The electron pair (de)coupling induced by a NH2.+ group to the bonds of a phenyl ring is investigated by means of polyelectron population analysis; the used density operators are built on the basis of both natural atomic orbitals (AOs) of ab initio-correlated molecular orbital (MO) wave functions, as well as the model orthogonal AOs of Parr-Pariser-Pople (PPP)+full configuration interaction (CI) ones. On the basis of two-electron, two-hole investigations, we calculate the electron pair localizations as well as the cooperation of opposite spins or charges to form bonds. The investigations of various electronic events referring to the simultaneous presence of a charge (+) and a radical center (.) lead to rules concerning the relative importance. of various resonance structures. As shown, the observed quinoid (de)coupling of electron pairs of the phenyl group is in the origin of the corresponding geometrical deformations. Given these results, we investigate the key electronic reasons responsible for the localized (bi- or tristable) or delocalized behavior of monooxidized aromatic polyamines.